Electrical resistor paste containing a small amount of charcoal

ABSTRACT

A resistor paste composition contains a small amount of powdered activated charcoal. The composition is applied to a ceramic substrate by screen printing, fired at an elevated temperature, and cooled to room temperature to form an electrical resistor element.

United States Patent 11 1 Aycioc k et al. v

[541 -ELECTRICAL RESISTOR PASTE Am. No.: 111,295

CONTAINING A SMALL AMOUNT OF 9 CHARCOAL V inventors: Thomas WL Aycock, Chelsea; John J. Wright, Fishkill, both of N.Y,

Assignee: International Business ,'Machines l Corporation, Armonk, NY. F1166: Feb. -1,' 1971 us. c1. ..252/502, 252/503, 2'52/506,

252/510, 252/511, 106/54 1m. 01.; ..1101b 1'/06,H0lb 3/08, HOlb 1/02,

- HOlb 1/04 Field 61s6a1-c11.....252/502, 510, 62.1, 503, 506,

[ 51 Jan. 16, 1973 561' Re ferences-Cited UNITED STATES PATENT S 241,529 5/1881 Craig ..'...,117/4 6CA- 1,747,649 2/1930 Ruben 3,010,842 11/1961 Ricker 3,248,345 I 4/1966 Mones et 211.. 3,390,104 6/1968 Miller et a1...

17/46 CA ..252/62.1 .....252/5l4 .....252/5l2 3,411,947 ll/l968 Block et a1. ..117 215 3,410,714 ll/l968 Jones ..1 17 46 CA 3,293,183 12 1966 Mathas .252/62.]

Primary Examiner-George F. Lesmes Assistant ExaminerJ. P. Brammer- Attorney-Hanifin & Jancin and David M. Bunnell 57 ABSTRACT A resistor paste composition contains a small amount of powdered activated charcoal. The composition is applied to a ceramic substrate by screen printing, fired at an elevated temperature, and cooled to room temperature to form an electricalre sistor element' 7 Claims, 5 Drawing Figures PATENTEDJAH 16 1975 FIG. 1

FIG. 3

FIG.

FIG. 4-

INVENTORS FIG. 5 THOMAS w. AYCOCK JOHN J. WRIGHT A TTORNEY SMALL AMOUNT OF CHARCOAL BACKGROUND OF THE INVENTION printed wiring. The functional components are active and passive electric circuit elements capable of performing useful functions or operations. Passive devices such as resistors are normally applied to the substrate by printing techniques.

The resistor compositions consist of a mixture of powdered insulating materials such as glass and powdered conductive materials such as metals and metal oxides, which may contain dopants to increase or decrease resistivity. The resistor compositions are applied to the module surface in the form of a paste by dispersing the powdered materials in a vehicle comprising a volatile solvent, resin, and wetting agent. The compositions are then fired at elevated temperatures. The firing dries off or burns out the vehicle to leave the fused insulating and conductive materials forming the resistor element.

With known resistor compositions it has not been found practical to initially print the resistors to the proper dimensions to achieve the desired resistance values. General practice, therefore, is to print and fire a resistor that is slightly oversized. The resistor is then trimmed, such as by a sand blasting process, to abrad away a portion of the resistor until the correct resistance value is obtained. Such resistor elements, compositions, and methods are described for example in U.S. Pat. Nos. 3,248,345; 3,345,158; 3,374,110; 3,390,l04;3,401,l26;3,4l l,947;and 3,414,641.

Although such methods and compositions have generally worked well to produce microminiaturized resistor elements, certain problems have arisen in that during the firing, blisters and/or craters sometimes form in the resistor film. This is particularly true in utilizing certain types of vehicles. The blistering produces void areas under the surface where the film I has drawn away from the module surface. This is visually apparent from an uneven surface appearance.

The craters adversely effect the electrical properties of the resistor element. The blistering, which results in weak spots in the resistor element, are a problem during the trimming process. When the trimming process is close to completion, a relatively large blistered section of resistor may break away. This results in a sudden change in resistance value which may place the resistor outside of the set tolerance limits. Usually when this occurs the entire module must be discarded as unusable. Also, even if the trimming operation is completed successfully, when the module is later encapsulated in a protective material, it has been found that the encapsulating material may cause a weakened or blistered portion of the resistor to separate from the module and the main body of the resistor element. This may place the resistance value beyond the acceptable tolerances and a completed module containing all of the elements in- I cluding the semiconductor chips must be discarded.

BRIEF DESCRIPTION OF THE INVENTION We have now found a new method of producing printed resistance elements whereby blister and crater formation are minimized.

In accordance with this invention a resistor paste composition comprises a mixture of a powdered conductive material, a powdered insulating material, an effective amount of powdered activated charcoal, and a vehicle comprising a volatile solvent and a resin binder.

The resistor is formed by depositing the paste composition on the surface of a substrate and solidifying it by firing it at an elevated temperature to drive off the charcoal and the vehicle.

The resistor element prepared by the above process has a relatively smooth surface texture with a minimum of blisters and craters.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a microphotograph of a resistor element made in accordance with a process of the invention.

FIG. 2 is a cross section of the resistor element of FIG. 1 taken along lines 22.

FIG. 3 is a microphotograph of a resistor element prepared without charcoal.

FIG. 4 is a cross sectional view of the resistor element of FIG. 3 taken along lines 4-4.

FIG. 5 shows the element of FIG. 4 after a trimming operation.

DETAILED DESCRIPTION The resistor paste is made up of a conductive solids mixture and an inert liquid vehicle. Generally, the solids mixture comprises about 50 to 83 percent by weight combined with 17 to 50 percent by weight of vehicle.

The conductive solids mixture or pigment is conventional and comprises, for example, a metal and/or oxide, a glass frit, and dopant ions. Metals and metal oxides include, for example, silver, indium, antimony, chromium, palladium, copper and mixtures thereof. Conventionally, the pigment will contain from about 30 to 50 percent by weight of the finely divided conductive material and 50 to percent by weight of the finely divided glass frit. The component materials have a particle size preferably in the range of l to 50 microns. Such pigment mixtures are well known in the art.

Included in the paste mixture is an amount of finely divided highly activated wood charcoal sufficient to substantially reduce the formation of craters and/or blisters in the fired resistor elements. Preferred amounts of charcoal range from about 0.4 percent to about 2 percent by weight of the total weight of paste. The range is not particularly critical but the amount of charcoal should be chosen to obtain a significant amount of blister and crater elimination without being excessive so as to adversely effect the flow properties of the paste (above about 0.2 percent but less than about 4.0 percent by weight based on the total weight of paste). The charcoal preferably has a particule size such that substantially all of it will pass through a 325 mesh U.S. standard screen. Suitable charcoals are highly purified or activated charcoals having a large surface area such as are used, for example, in packing gas purification columns or pharmaceutical grade highly refined wood charcoal.

The vehicles are conventional and usually comprise a non polar solvent combined with a resin, and sometimes a wetting agent. The resin in combination with the solvent gives the resistor composition the desired flow characteristics. The composition-must be fluid enough to allow the employment of silk screening techniques but sufficiently firm after being printed or transferred to the substrate to maintain its physical dimensions prior to drying. The wetting agents are employed to aid in dispersing and maintaining the solid pigment materials in suspension in the vehicle. The wetting agents also aid in the screening characteristics. Conventionally the vehicles constitute '17 to 50 percent by weight and more preferably 20 to 30 percent by weight of the total weight of resistor paste composition.

The resin components are conventional and include for example natural gums, cellulosic materials and synthetic resins. Suitable volatile solvents are those having a relatively low vapor pressure at room temperature and a relatively high vapor pressure at elevated temperatures. Solvents which are commonly employed are higher boiling paraffins, cycloparaffins, terpineol and aromatic hydrocarbons such as ethyl naphthalene, phenylcyclohexane, and mixtures thereof; or one or more of the mono and di-alkyl ethers of diethylene glycol or their derivatives such as diethylene glycol monobutyl ether acetate.

Surfactants or dispersing agents suitable for use are, for example, organic derivatives such as polyoxyethylene alcohol non-ionic surfactants, alkylaryl sulfonate's, and fatty acid esters.

The resin generally comprises from about 20 to 80 percent by weight of the total weight of the vehicle and the volatile liquid from about 80 to 20 percent by weight of the total weight of the vehicle. The solvent preferably constitutes 50 to 70 percent byweight of the total weight of the vehicle.

In preparing the resistor paste, conventional mixing procedures are employed. Usually the conductive solids mixture is thoroughly mixed by any suitable conventional dry mixing procedure such as, for example, by shaking in a closed container using a paint mixer. The vehicle and pigment mixture are then thoroughly mixed. A mixing procedure using a three roll mill such as is described, for example, in U.S. Pat. No. 3,414,641 7 can be employed.

The charcoal can be added either to the dry conductive mixture prior to blending with the vehicle or it can be added to the resistor paste mixture.

, Resistor elements are formed by depositing the paste onto the substrate using conventional techniques, for example, by screening, drying the paste in a furnace at temperatures of from about 100 to 300 C. and then firing the paste at elevated temperatures up to about 760 C. to driveoff the vehicle and fuse the conductive and insulating powders. It has been found that the firing process also removes substantially all of the charcoal so that the resistance properties are not affected by the presence of any residual charcoal. The presence of the charcoal during the firing process produces a relatively smooth, crater and void free resistor element which has excellent electrical properties and which will adhere to the substrate during trimming operations and during use.

The foregoing and other features of the invention are further illustrated by, but are not intended to be limited to the following examples wherein parts are parts by weight unless otherwise indicated.

EXAMPLE 1 A conductive mixture was prepared from the following:

Parts by Wt. Silver 16.0 Doped Palladium Oxide 21.0 Borosilicate Glass 58.0 Colloidal Silica 3.0 Activated Wood Charcoal (Darco G-6, Atlas Chemical Industries) 2.0 100.0

The powders were mixed for 15 minutes in a closed container on a paint shaker.

A vehicle for the resistor paste was made up of the following constituents:

. Parts by Wt. Butyl carbatol acetate 90.0 Ethylcellulose (N-22, Hercules Powder Co.) 10.0 Oleyl Sarcosin Surfactant (Sarkosyl O, Geigy Chemical Co.) 0.5

These ingredients were agitated in a closed high kinetic energy dispersing unit after which 27.6 grams of the vehicle and 0.6 grams of colloidal silica were added to grams of the dry conductive mixture and thoroughly mixed. The paste was then passed through a three-roll mill six times. Portions of the paste were screened onto a ceramic module substrate, dried for aboutS minutes at 200 C. and then fired in a zone furnace having a maximum temperature of about 760 C. for a total time of about 35 minutes to form a resistor element having a relatively smooth surface.

EXAMPLE 2 A resistor element was prepared according to the formulary and method of Example 1 except that no charcoal was added to the dry mix. After firing, the surface of the resistor element had a cratered and blistered appearance.

FIGS. 1 through 4 illustrate typical resistor elements prepared by the processes of Examples 1 and 2. The resistor element 11, formed by the composition and process of the invention on the surface of substrate 13 betweenconductor lines 15, has a relatively smooth surface with little if any blistering or cratering being observable (FIGS. 1 and 2). Also from the cross-section illustrated in FIG. 2 resistor element 11 has a solid cross section. In contrast to this, as is shown in FIGS. 3 and 4, the resistor element 21 on substrate 23 between conductive elements 25 prepared in accordance with Example 2 has a rough uneven surface appearance with craters and blistering being visually apparent. This is further illustrated in FIG. 4 where the cross section, of resistor 21 has a series of voids 27 or blisters where the resistor 21 has pulled away from the surface 29 of substrate The deleterious effects of the blistering of the resistor element prepared in accordance with Example 2 are illustrated in the cross-section shown in FIG. 5

wherein in order to trim the resistor to the correct resistance value a portion of the resistor element has been abraded away from the substrate. 23. It can be seen that the trimmed edge 31 of resistor element 21 in blistered portion 27 is not in contact or supported by substrate 23. Consequently, during further trimming or after the resistor element and its associated module have been encapsulated in a protective material the blistered portion is prone to break away from the body of the resistor element. This usually results in placing the resistance of the resistor 21 beyond the electrical tolerances of the device.

The fired resistor elements were analyzed for residual carbon content with a somewhat higher percent carbon (0.25 vs. less than 0.0l per cent average of two determinations) found in the element prepared from the paste with the activated charcoal. The electrical properties of the resistor element of Examples 1 and 2 are illustrated in Table 1 below:

TABLE 1 Resistor Element Resistor Element of Example 1 of Example 2 Power Drift 0.68% -H).O33% Power Limits :1 il 25% Moisture Drift +2.l9% +0.087%

*where values are change in ohms.

These electrical properties are comparable. Therefore, the novel resistor element is not adversely affected by any residue resulting from the presence of the activated charcoal during the resistor preparation.

EXAMPLE 3 A resistor element was prepared using the formula and procedure of Example 1 except that a different vehicle was employed. The vehicle was made up of the following constituents:

Parts by Wt. Terpineol Solvent 90.0 Ethylcellulose (N-7 Hercules Power Co.) 10.0 Oleyl Sarcosin Surfactant (Sarkosyl O. Geigy Chem. Co.) 0.5

The resistor element prepared from the resistor paste by the procedure followed in Example 1 had excellent electrical properties and a relatively smooth surface contour which had few if any craters or voids and a substantially solid cross section.

To further illustrate the process of the invention a series of resistor pastes were formulated using the dry conductor mixture and vehicle as prepared in Example 1 except that the paste was first mixed without the charcoal additive and to three portions of the mixed paste were added varying amounts as shown in Table 2 below of a pharmaceutical grade of activated charcoal which was ground to pass through a 325 mesh screen (U.S. standard sieve series). After the addition of the charcoal to each portion of paste, the paste was milled six more times through the three roll mill. Resistor elements were prepared in accordance with the procedure of Example 1. The amount of blistering was visually TABLE 2 Charcoal (by wt. of total paste) 1 Blistering & Cratering Excessive Excessive Reduction in Blistering & Cratering 90% Reduction in Blistering & Cratering E" 9 0 AM The results observed are recorded in Table 2 above where it can be seen that increasing amounts of charcoal resulted in increased reduction in the blistering. The optimum amount of charcoal to be added to any particular paste formulation can readily be determined by one skilled in the art.

The foregoing has described a resistor composition which greatly reduces the deleterious effects resulting from blistering and cratering which previously could occur during the firing of the resistor paste to form the fused resistor elements. This reduction in blistering greatly reduces the number of modules which must be discarded during manufacture or use.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

We claim:

1. In a resistor paste containing about 50 to 83 percent by weight of paste ofa solid portion, said solid portion comprising about 30 to 50 percent by weight of the solid portion of a powdered conductive material selected from the group consisting of metals, metal oxides and mixtures thereof and about 50 to 70 percent by weight of solid portion of a about 17 to 50 percent by weight of paste of a vehicle, said vehicle comprising about 20 to percent by weight of vehicle of an organic polymer and about 20 to 80 percent by weight of vehicle of a volatile non-polar solvent,

the improvement which comprises including in said paste, from about 0.2 to 4.0 percent by weight of the total weight of paste, of an activated charcoal having a particle size such that it will pass through a 325 mesh U.S. standard screen.

2.The paste of claim 1 wherein said solid portion includes dopant ions.

3. The paste of claim 1 wherein said charcoal is present in an amount of about 0.4 to 2.0 percent by weight of the total weight of paste.

4. The paste of claim 3 wherein said charcoal is a pharmaceutical grade highly refined wood charcoal.

5. The paste of claim 1 wherein said vehicle includes a wetting agent.

6. The paste of claim 1 wherein said solid portion includes colloidal silica.

7. The resistor composition of claim 1 wherein said powdered conductive material is lithium doped paladium oxide said powdered glass insulating material is a borosilicate glass, and said vehicle compises a volatile solvent and ethyl cellulose.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Patent No. 3,711,428 Dated January 16, 1973 Inventor(s) Thomas Aycock et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6,' line 59, after "a" insert powdered glass insulating material, said paste also containing Signed and sealed this 19th day of March, 1974.

(SEAL) 'Atte'st:

EDWARD'M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) USCOMMfDC BO376-P69 U.S. GOVERNMENT PRINTING OFFICE: 1969 0-366-35,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,711,428 Dated January 16, 1973 1nvmumr(s) Thomas W. Aycock et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below Column 6; line 39, after "a" insert powdered glass insulating material, said paste also containing Signed and sealed this 19th day of March 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) USCOMM Dc 60316 \LST GOVERNMENT PRINTING OFFI CE: [9G9 0-355-33L 

2. The paste of claim 1 wherein said solid portion includes dopant ions.
 3. The paste of claim 1 wherein said charcoal is present in an amount of about 0.4 to 2.0 percent by weight of the total weight of paste.
 4. The paste of claim 3 wherein said charcoal is a pharmaceutical grade highly refined wood charcoal.
 5. The paste of claim 1 wherein said vehicle includes a wetting agent.
 6. The paste of claim 1 wherein said solid portion includes colloidal silica.
 7. The resistor composition of claim 1 wherein said powdered conductive material is lithium doped paladium oxide said powdered glass insulating material is a borosilicate glass, and said vehicle compises a volatile solvent and ethyl cellulose. 